1. Field of the Invention
The present invention relates to polarizing elements, particularly to polarizing lenses, more particularly to polarizing goggles, eye shields or ophthalmic lenses, and to novel compositions of polarizing layers in ophthalmic lenses comprising melanin and substituted/derivatized melanin.
2. Background of the Art
Optical polarizing film is widely used for glare reduction and for increasing optical contrast in such products as sunglasses and Liquid Crystal Displays (LCD). One of the most commonly used types of polarizers for these applications is a dichroic polarizer which absorbs light of one polarization and transmits light of the orthogonal polarization. One type of dichroic polarizer is made by incorporating a dye into a polymer matrix which is stretched in at least one direction. Dichroic polarizers may also be made by uniaxially stretching a polymer matrix and staining the matrix with a dichroic dye. Alternatively, a polymer matrix may be stained with an oriented dichroic dye. Dichroic dyes include anthraquinone and azo dyes, as well as iodine. Many commercial dichroic polarizers use polyvinyl alcohol as the polymer matrix for the dye.
Another type of polarizer is a reflective polarizer which reflects light of one polarization and transmits light of another orthogonal polarization. One type of reflective polarizer is made by forming a stack of alternating sets of polymer layers, one of the sets being birefringent to form reflective interfaces in the stack. Typically, the indices of refraction of the layers in the two sets are approximately equal in one direction so that light polarized in a plane parallel to that direction is transmitted. The indices of refraction are typically different in a second, orthogonal direction so that light polarized in a plane parallel to the orthogonal direction is reflected.
One measure of performance for polarizers is the extinction ratio. The extinction ratio is the ratio of a) light transmitted by the polarizer in a preferentially transmitted polarization state to b) light transmitted in an orthogonal polarization state. These two orthogonal states are often related to the two linear polarizations of light. However, other types of orthogonal states, such as, left and right-handed circular polarizations or two orthogonal elliptical polarizations may also be used. The extinction ratios of dichroic polarizers vary over a wide range depending on their specific construction and target application. For example, dichroic polarizers may have extinction ratios between 5:1 and 3000:1. Dichroic polarizers used in display systems typically have extinction ratios which are preferably greater than 100:1 and even more preferably greater than 500:1.
Dichroic polarizers typically absorb light in the non-transmission polarization. However, dichroic polarizers also absorb some of the light having the high transmission polarization. The amount of this absorption depends on the details of the construction of the polarizer and the designed extinction ratio. For high performance display polarizers, such as those used in LCDs, this absorption loss is typically between about 5 and 15%. The reflectivity of these polarizers for light having the absorption (i.e., low transmission) polarization tends to be small. Even with surface reflections included, this reflectivity is typically less than 10% and usually less than 5%.
Reflective polarizers typically reflect light having one polarization and transmit light having an orthogonal polarization. Reflective polarizers often have incomplete reflectivity of the high extinction polarization over a wavelength region of interest. Typically, the reflectivity is greater than 50% and is often greater than 90% or 95%. A reflective polarizer will also typically have some absorption of light having the high transmission polarization. Typically, this absorption is less than about 5 to 15%. Structures and materials for the manufacture of light polarizing films with polyvinyl alcohol (PVA) and dichroic dyes include at least U.S. Pat. Nos. 4,859,039, 4,992,218, 5,051,309, 5,071,906, 5,326,507, 5,582,916, and 6,113,811. These patents are incorporated herein in their entirety for their disclosure of materials, ancillary materials, and processes and structures for polarizing elements and layers.
Ophthalmic lenses are also well known for having dyes and/or pigments therein for absorbing electromagnetic radiation, particularly electromagnetic radiation such as visible radiation, ultraviolet radiation and any other ionizing radiation. The use of Melanin as a pigment in ophthalmic lenses for absorbing electromagnetic radiation is taught in U.S. Pat. Nos. 5,112,883, 5,047,447, 5,036,115, and 4,698,374, which patents are incorporated herein by reference in their entirety for their teachings of melanin, the properties of melanin, their utility in radiation absorbing layers, ancillary materials, processes of manufacture, structures and the like containing melanin.
It is generally known and accepted in the polarizing art that structures use only a dichroic dye or combinations of dichroic dyes to get the desired color and absorption of light in the PVA polarizers. This is specifically disclosed and considered in U.S. Pat. No. 4,859,039. It is noted therein that, (column 3 line 35) xe2x80x9cMixing various dyes is necessary, which respectively have different absorption and polarization characteristics at different wavelength regions within the range of 400-700 nm. One dye only is hardly able to provide a polarizing film, over the entire visible light region of 400-700 nm, showing the same absorption characteristics, i.e., neutral gray, and having a high polarizing coefficient.xe2x80x9d Then it is noted that, (column 2 line 27) xe2x80x9cAny organic direct dyes may be employed in the present invention, as long as the dichroism ratio is large.xe2x80x9d
A dichroic dye can be blended with a non-dichroic dye (Melanin, synthetic melanin and melanin derivatives) and still maintain a commercially acceptable level of polarizing efficiency of  greater than 97% (as later defined in this text). Efficient polarizers exhibit polarizing efficiencies of  greater than 95% and the best are  greater than 98%. Since Melanin has unique features with respect to absorption of wavelengths of light, melanin is very useful to have in combination with a polarizer that takes advantage of this absorption. The blending can be done by a mixture (preferably an aqueous mixture) of Melanin and a dichroic dye in a ratio sufficient to achieve the desired color and polarizing effect. Alternatively, the PVA film may be first passed through the solution of the dichroic dye and then the solution of Melanin dye or the Melanin dye and then the dichroic dye. It was found that an efficient polarizer layer can be made where the dichroic dye contributes from 10-90% of light absorption (preferably from 30-70% or 35-65%, or 40-60%, e.g., one half) and the melanin dye contributed the remaining amount of the desired absorption of light in the region of 400-700 nm. To make the polarizing sheet, for example, the oriented PVA film is passed through the aqueous mixtures as described in the examples in 4,859,039. The films may be used in providing polarizing properties to any reflective or transmissive element such as reflecting mirrors, goggles, eye shields, swimming or diving goggles, sunglasses, ophthalmic lenses and the like.